1. Field of the Invention
This invention relates to a process for producing a lithium manganese oxide suitable for use as a 3 volt cathode (positive electrode) material of lithium ion secondary (rechargeable) batteries. More particularly, this invention relates to a process for producing a lithium manganese oxide which has a large specific surface area and a large tap density and is suitable for use as a 3 volt high energy density cathode material of non-aqueous electrolyte (organic electrolyte) lithium ion secondary batteries easily by use of less costly starting materials,
2. Prior Art
Backed by the recent demands for small sized and high performance electronic appliances, high voltage and high energy density lithium ion secondary batteries are already in use as their power sources in some fields. Lithium ion secondary batteries are also expected to be promising as power sources for large scale electric power storage systems or electricmobiles to cope with world-wide decrease in resources and environmental pollution.
A lithium cobalt oxide, LiCoO.sub.2, has been so far used as a high performance cathode material of non-aqueous electrolyte lithium ion secondary batteries. However, since the amount of cobalt in natural resources is small and it is expensive, a lithium nickel oxide or a lithium manganese oxide has attracted attention as a cathode material of non-aqueous electrolyte lithium ion secondary batteries in the next generation.
Since the amount of mangenese is rich in natural resources and it is less expensive, the study for practical use of a 4 volt spinel lithium manganese oxide or a 3 volt layered lithium manganese oxide has been conducted. Among these oxides, the latter lithium manganese oxide has a theoretical charge-discharge capacity of 286 mAh/g and it is expected that the oxide has a charge-discharge capacity larger than the lithium cobalt oxide (LiCoO.sub.2).
According to A. R. Armstrong et al., Nature, Vol. 381, Jun. 6, 1996, pp. 499-500, the solid-state reaction between stoichiometric quantities of sodium carbonate and manganese (III) oxide under an argon atmosphere provides a stoichiometric sodium manganese oxide (NaMnO.sub.2) and the ion exchange of the oxide provides a stoichiometric lithium manganese oxide (LiMnO.sub.2) which has a layered structure.
However, lithium manganese oxides including LiMnO.sub.2 for use as a cathode material of lithium ion secondary batteries have been hitherto produced by a dry calcining process or a solid-state synthesizing process wherein a powder of a lithium compound such as lithium hydroxide or lithium carbonate is dry-mixed with a powder of a manganese oxide and then the powder mixture is calcined at a temperature as high as 500.degree. C. or more, as described in Japanese Patent Application Laid-open No. 8-37006 or No. 8-37027.
In general, a solid-state reaction using two solid powders as reactants is carried out by heating the solid powders at high temperatures capable of moving ions or atoms constituting the reactants, and mutually diffusing the ions or the atoms between the solid phases of these two solid powders. Accordingly, the smaller the diameter of the particle of the solid reactants, the shorter the diffusion distance and the larger the diffusion surface. Thus, a dense product having a uniform composition can easily be obtained.
However, in the production of a desired oxide by the dry calcining method, it is difficult to obtain a uniform powder mixture having a particle diameter of a submicron level by dry-mixing a powder of a manganese oxide with a powder of a lithium compound since the manganese oxide and lithium compound have a particle diameter of from several microns to tens of microns, and in particular, lithium hydroxide has a particle diameter of more than about 100 microns or more. Further, since the starting materials and the resulting final products have a low thermal conductivity, it is necessary to calcine the powder mixture at high temperatures for many hours, and in addition, it is necessary to repeat such high temperature calcination in order to obtain a lithium manganese oxide by dry calcination of the powder mixture.
Thus, when the calcination temperature and time are inappropriate in the calcination of the powder mixture, undesired by-products are often formed or the surface properties of the resultant product undesirably change with the result that a high performance cathode material for lithium ion secondary batteries are not obtained.
A further method, e.g., a melt impregnation method, is also known wherein lithium nitrate is heated and melted, and manganese dioxide is impregnated with the melted lithium compound, followed by calcining the product at high temperatures (J. Power Sources, 54 (1995), 483-486). However, the method also needs to repeat high temperature calcination for many hours.